1. Field of the Invention
This invention relates to a heat and radiation-resistant optical fiber.
2. Prior Art
As is well known in the art, optical fibers have now been extensively used for communication, image-transmitting and illumination purposes, and such optical fibers comprise at least one optical fiber element for transmitting light which element is made of a glass material such as multi-component glass and silica glass. Those optical fibers comprising at least two optical fiber elements are often referred to as "multiple fiber" in the trade. The optical fiber element has a protective coating formed on its surface. Without such a coating, a fine defect or damage present on the surface of the optical fiber element would grow to such a size that the mechanical strength of the fiber element is finally reduced below an acceptable level. Such a coating will also increase tensile strength and bending strength and enhance an environmental resistance. Various such coatings have heretofore been proposed. However, such coatings are soon deteriorated when the optical fibers are installed in a nuclear reactor since the coatings fail to offer sufficient resistance to intense heat and radioactivity. As a result, the mechanical strength of the optical fibers is reduced, and the fiber is finally subjected to rupture.
It has therefore been proposed to apply a multi-layer coating to the optical fiber element or elements to provide an optical fiber for use in a severe environment such as in a nuclear reactor. This procedure has been found disadvantageous, however, in that the resultant optical fiber is rendered less flexible because of the provision of the multi-layer coating, so that such an optical fiber can not be easily handled and installed.
To overcome this difficulty, it has also been proposed to apply a coating of a fluorine-based resin, such as FEP, TFE, TETZEL and KF, onto an optical fiber element or elements through an inner layer of silicone rubber. This approach has also been found disadvantageous, however, in that when such a coating is subjected to radiation, it produces hydrofluoric acid which tends to corrode the surface of the optical fiber element to reduce its mechanical strength. To eliminate this disadvantage, it has been proposed to use as the inner layer silicone rubber containing metal oxides, but satisfactory effects could not be achieved.